1. Field of the Invention
The present invention relates to coated films comprising alpha-olefin polymers, such as polypropylene consisting essentially of recurring propylene units and having a substantial crystalline polypropylene content, and in particular isotactic polypropylene, which are easily thermoweldable and which have an improved resistance to sticking against the welding bars of automatic and semi-automatic machines conventionally used for the manufacture of containers, envelopes, bags, vessel and, in general, other manufactured articles useful in the packaging industry.
2. The Prior Art
Patents assigned to Montedison S.p.A. describe methods for preparing films of the alpha-olefin polymers made of macromolecules having, prevailingly, stereoregular structure, isotactic, and, in particular, of polypropylene substantially consisting of macromolecules of isotactic structure and obtained by polymerizing propylene in the presence of stereospecific catalysts.
Because of their mechanical, protective and optical properties, films obtained from crystalline polyolefins, in particular polypropylene, are materials the widest field of application of which is in the packaging industry.
One difficulty encountered in adapting such films for use in the packaging art is that these films made of polyolefins, and in particular of polypropylene, cannot be processed or transformed using conventionally available automatic and semi-automatic machines. This is due to the fact that the automatic and semi-automatic machines have been designed and built keeping in mind the characteristics of the packaging material most widely used up to now, that is, the cellophane film which, as is quite known, is insensitive to heat until reaching its inflammable temperature.
The use of polyolefinic and in particular of polypropylene films on automatic and semi-automatic thermowelding devices mounted on the standard packaging machines, involves a considerable reduction of the productivity due to the melting of the film in the points of contact with the welding elements, or to the sticking to these latter or to ruptures, making it practically impossible to maintain a commercially reasonable operating speed of the machine.
It is known to improve the thermoweldability characteristics of the synthetic polymer films, by coating the film with materials capable of imparting said characteristics to the films.
In general, the known methods involve coating the supporting film by extruding a welding layer on the supporting film, in the molten state, (extrusion coating). Still another method is the so-called "lamination" of two films against each other, with or without the interposition of adhesives. A further method consists in spreading a solution of the coating agent in a suitable solvent, on the supporting film. The coating agent may be coupled to an anchoring agent (primer) which facilitates the adhesion of the coating to the supporting film.
Often the primer is substituted by, or used with, a pre-treatment of the supporting film with chemical agents, electrical non-piercing discharges, a flame, or other suitable pre-treatment.
As materials suitable for the coating of polyolefinic films there have been described and suggested numerous compounds, prevailingly polymeric compounds, which, in general, are applied in the form of solutions in an organic solvent. The use of organic solvents, which successively are removed from the coated film by evaporation, makes necessary the installation of suitable equipment for the recovery of the solvents and of devices that will keep the work places free of vapors.
Moreover, the use of organic solvents involves suitable preventive measures against pollution of the environmental atmosphere and of the liquid discharged effluents.
There have also been suggested polymeric coating materials applicable to the films in the form of aqueous dispersions.
Thus, for instance, U.S. Pat. No. 3,753,769 discloses the use of an acrylic interpolymer, as coating material, applicable by an aqueous alkaline solution, such as for instance by an ammonia solution.
The interpolymer disclosed in this U.S. Pat. No. 3,753,769 consists essentially of: (a) from about 2 to about 15 parts by weight of an alpha-beta-mono-ethylenically unsaturated carboxylic acid, such as acrylic acid, methacrylic acid, maleic acid, crotonic acid etc., and (b) from about 85 to about 98 parts by weight of neutral monomer esters, preferably comprising (i) an alkylacrylate such as methyl-, ethyl-, or butyl-acrylate and (ii) an alkyl methacrylate such as methyl-methacrylate or an ethyl-methacrylate. The monomer componenets are employed in a ratio such that the alkylmethacrylate monomer is present in an amount of at least 10% by weight, and preferably from about 20 to about 80% by weight, with respect to the total terpolymer composition, and the alkylacrylate monomer component in amounts of at least 10% by weight, and preferably from about 80 to about 20% by weight, of the total terpolymer composition.
The possibility to add the coating compositions onto the polyolefin films from aqueous solutions or dispersions allows to achieve a substantial saving on the process because it does not require the use of expensive devices for the recovery of the solvents and for the solution of environmental hygene and pollution problems, and, also, it makes the process less complex.
However, the films coated with the above disclosed interpolymers show excellent values of weld resistance, but have a poor and unsatisfactory sticking resistance to the welding bars.